Field of the Invention
The present invention relates to a method for efficiently transmitting Reference Signals (RSs) in a Multiple Input Multiple Output (MIMO) communication system, under an environment where antennas are added to an existing system.
Discussion of Related Art
Long Term Evolution (LTE) Physical Structure
3rd Generation Partnership Project (3GPP) supports a type-1 radio frame structure for application in Frequency Division Duplex (FDD) and a type-2 radio frame structure for application in Time Division Duplex (TDD).
FIG. 1 illustrates the type-1 radio frame structure. A type-1 radio frame includes 10 subframes each having two slots.
FIG. 2 illustrates the type-2 radio frame structure. A type-2 radio frame includes two half frames each having 5 subframes, a Downlink Pilot Time Slot (DwPTS), a Guard Period (GP), and an Uplink Pilot Time Slot (UpPTS). Each subframe includes two slots. The DwPTS is used for initial cell search, synchronization, or channel estimation in a User Equipment (UE), whereas the UpPTS is used for channel estimation in a Base Station (BS) and uplink transmission synchronization in a UE. The GP is a period between a downlink and an uplink, for eliminating interference with the uplink caused by a multi-path delay of a downlink signal. Irrespective of the types of radio frames, each subframe includes two slots.
FIG. 3 illustrates an LTE downlink slot structure. Referring to FIG. 3, a signal transmitted in each downlink slot may be described by a resource grid with NRBDL NscRB subcarriers by NsymbDL Orthogonal Frequency Division Multiplexing (OFDM) symbols. NRBDL represents the number of downlink Resource Blocks (RBs) and NscRB represents the number of subcarriers per RB. NsymbDL represents the number of OFDM symbols in the downlink slot.
FIG. 4 illustrates an LTE uplink slot structure. Referring to FIG. 4, a signal transmitted in each uplink slot may be described by a resource grid with NRBUL NSCRB subcarriers by NsymbUL Single Carrier-Frequency Division Multiple Access (SC-FDMA) symbols. NRBUL represents the number of uplink RBs and NscRB represents the number of subcarriers per RB. NsymbUL represents the number of SC-FDMA symbols in the uplink slot.
A Resource Element (RE) is a resource unit indicated by index (a, b) in the downlink and uplink slots, occupying one subcarrier by one OFDM or SC-FDMA symbol. Here, a denotes a frequency index and b denotes a time index.
FIG. 5 illustrates an example of a downlink subframe structure. Referring to FIG. 5, the first three OFDM symbols of a first slot form a control region allocated to control channels, and the other OFDM symbols of the first slot form a data region allocated to a Physical Downlink Shared CHannel (PDSCH) in a subframe. In 3GPP LTE, the downlink control channels include, for example, a Physical Control Format Indicator CHannel (PCFICH), a Physical Downlink Control CHannel (PDCCH), and a Physical Hybrid Automatic Repeat reQuest (ARQ) Indicator CHannel (PHICH).
The PCFICH, which is transmitted in the first OFDM symbol of a subframe, carries information about the number of OFDM symbols used for the control channels of the subframe. The PHICH delivers a Hybrid ARQ (HARQ) ACKnowledgment/Negative ACKnowledgment (ACK/NACK) signal as a response for an uplink transmission. The PDCCH delivers control information called Downlink Control Information (DCI) which includes downlink or uplink scheduling information or information about an uplink transmit power control command for a user group. Specifically, the PDCCH may carry transport format information, a resource assignment of a Downlink Shared CHannel (DL-SCH), paging information on a Paging CHannel (PCH), system information on the DL-SCH, a resource assignment of an upper-layer control message such as a random access response transmitted on a PDSCH, a set of transmit power control commands for individual UEs of a UE group, and Voice over Internet Protocol (VoIP) activation information. A plurality of PDCCHs may be transmitted in the control region. A UE may monitor the plurality of PDCCHs. A PDCCH is transmitted in a set of one or more consecutive Control Channel Elements (CCEs). The CCEs are logical allocation units used to provide a coding rate to the PDCCH based on a radio channel state. A CCE is a plurality of RE groups. The format of the PDCCH and the number of bits available to the PDCCH are determined according to the correlation between the number of CCEs and a coding rate that the CCEs provide. A BS determines the format of the PDCCH based on DCI transmitted to a UE on the PDCCH and attaches a Cyclic Redundancy Check (CRC) to the control information.
The CRC is masked with an Identifier (ID) specific to the usage of the PDCCH or the user (a Radio Network Temporary Identifier (RNTI)). If the PDCCH is destined for a particular UE, the CRC may be masked with an ID specific to the UE (e.g. a Cell-RNTI (C-RNTI)). If the PDCCH is used for a paging message, the CRC may be masked with a paging indicator ID (e.g. a Paging-RNTI (P-RNTI)). If the PDCCH is used to carry system information, particularly System Information Blocks (SIBs), the CRC may be masked with a system information ID and a System Information-RNTI (an SI-RNTI). To indicate a random access response to a random access preamble received from a UE, the CRC may be masked with a Random Access-RNTI (a RA-RNTI).
FIG. 6 illustrates an uplink subframe structure. Referring to FIG. 6, an uplink subframe may be divided into a control region and a data region in the frequency domain. The control region is allocated to Physical Uplink Control CHannels (PUCCHs) for carrying uplink control information. The data region is allocated to a Physical Uplink Shared CHannel (PUSCH) for transmitting data. To maintain a single carrier property, a UE does not transmit a PUCCH and a PUSCH simultaneously. A pair of RBs are allocated to a PUCCH for a UE in a subframe. The RBs respectively occupy different subcarriers in two slots, with frequency hopping at the boundary between the slots.
Definition of MIMO
The term “MIMO” is short for Multiple Input Multiple Output. Beyond conventional schemes using a single Transmit (Tx) antenna and a single Reception (Rx) antenna, MIMO uses a plurality of Tx antennas and a plurality of Rx antennas to thereby increase the transmission and reception efficiency of data. With the use of multiple antennas at a transmitter or a receiver, MIMO seeks to increase capacity or improve performance in a wireless communication system. The term “MIMO” is interchangeable with “multiple antenna”.
The MIMO technology does not depend on a single antenna path to receive an entire message. Rather, it completes the message by combining data fragments received through a plurality of antennas. Because MIMO may increase data rate within a certain area or extend system coverage at a given data rate, it is considered as a promising future-generation mobile communication technology that may find its use in a wide range including mobile terminals, relays, etc. With the growth of data communication, MIMO is attracting attention as a future-generation technology that may overcome a limit on transmission capacity that is almost reached due to the increased data communication.
MIMO System Model
FIG. 7 illustrates the configuration of a typical MIMO communication system. Referring to FIG. 7, a simultaneous increase in the number of Tx antennas of a transmitter to NT and in the number of Rx antennas of a receiver to NR increases a theoretical channel transmission capacity in proportion to the number of antennas, compared to use of a plurality of antennas at only one of the transmitter and the receiver. Therefore, transmission rate and frequency efficiency are remarkably increased. Given a maximum transmission rate Ro that may be achieved in case of a single antenna, the increase of channel transmission capacity may increase the transmission rate, in theory, to the product of Ro and Ri in case of multiple antennas. Ri is a transmission rate increase rate.Ri=min(NT, NR)  [Equation 1]
For instance, a MIMO communication system with four Tx antennas and four Rx antennas may achieve a four-fold increase in transmission rate theoretically, relative to a single-antenna system. Since the theoretical capacity increase of the MIMO system was proved in the middle 1990's, many techniques have been actively studied to increase data rate in real implementation. Some of the techniques have already been reflected in various wireless communication standards for 3rd Generation (3G) mobile communications, future-generation Wireless Local Area Network (WLAN), etc.
Concerning the research trend of MIMO, active studies are underway in many respects of MIMO, inclusive of studies of information theories related to calculation of multi-antenna communication capacity in diverse channel environments and multiple access environments, studies of measuring MIMO radio channels and MIMO modeling, studies of time-space signal processing techniques to increase transmission reliability and transmission rate, etc.
To describe a communication scheme in a MIMO system in detail, the following mathematical model may be used. It is assumed that there are NT Tx antennas and NR Rx antennas as illustrated in FIG. 7. Regarding a transmission signal, up to NT pieces of information can be transmitted through the NT Tx antennas, as expressed as the following vector.s=└s1, s2, . . . , sNT┘T  [Equation 2]
A different transmit power may be applied to each piece of transmission information s1, s2, . . . , sNT. Let the transmit power levels of the transmission information be denoted by P1, P2, . . . , PNT, respectively. Then the power-controlled transmission information ŝ may be given as [Equation 3].ŝ=[ŝ1, ŝ2, . . . , ŝNT]T=[P1s1, P2s2, . . . , PNTsNT]T  [Equation 3]
ŝ may be expressed as a diagonal matrix P of transmit power.
                              s          ^                =                                            [                                                                                          P                      1                                                                                                                                                                                                                                                                                0                                                                                                                                                                                                                P                      2                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    ⋱                                                                                                                                                                                          0                                                                                                                                                                                                                                                                                  P                                              N                        T                                                                                                        ]                        ⁡                          [                                                                                          s                      1                                                                                                                                  s                      2                                                                                                            ⋮                                                                                                              s                                              N                        T                                                                                                        ]                                =                      P            ⁢                                                  ⁢            s                                              [                  Equation          ⁢                                          ⁢          4                ]            
Meanwhile, actual NT transmitted signals x1, x2, . . . , xNT may be configured by applying a weight matrix W to the power-controlled information vector ŝ. The weight matrix W functions to appropriately distribute the transmission information to the Tx antennas according to transmission channel statuses, etc. These transmitted signals x1, x2, . . . , xNT are represented as a vector X, which may be determined as
                    x        =                              [                                                  ⁢                                                                                x                    1                                                                                                                    x                    2                                                                                                ⋮                                                                                                  x                    i                                                                                                ⋮                                                                                                  x                                          N                      T                                                                                            ]                    =                                                    [                                                                  ⁢                                                                                                    w                        11                                                                                                            w                        12                                                                                    …                                                                                      w                                                  1                          ⁢                                                                                                          ⁢                                                      N                            T                                                                                                                                                                                                  w                        21                                                                                                            w                        22                                                                                    …                                                                                      w                                                  2                          ⁢                                                                                                          ⁢                                                      N                            T                                                                                                                                                                          ⋮                                                                                                                                                                          ⋱                                                                                                                                                                                                                                      w                                                  i                          ⁢                                                                                                          ⁢                          1                                                                                                                                    w                                                  i                          ⁢                                                                                                          ⁢                          2                                                                                                            …                                                                                      w                                                  i                          ⁢                                                                                                          ⁢                                                      N                            T                                                                                                                                                                          ⋮                                                                                                                                                                          ⋱                                                                                                                                                                                                                                      w                                                                              N                            T                                                    ⁢                          1                                                                                                                                    w                                                                              N                            T                                                    ⁢                          2                                                                                                            …                                                                                      w                                                                              N                            T                                                    ⁢                                                      N                            T                                                                                                                                              ]                            ⁡                              [                                                                                                                              s                          ^                                                1                                                                                                                                                                          s                          ^                                                2                                                                                                                        ⋮                                                                                                                                                    s                          ^                                                J                                                                                                                        ⋮                                                                                                                                                    s                          ^                                                                          N                          T                                                                                                                    ]                                      =                                          W                ⁢                                                                  ⁢                                  s                  ^                                            =                              W                ⁢                                                                  ⁢                P                ⁢                                                                  ⁢                s                                                                        [                  Equation          ⁢                                          ⁢          5                ]            
Wij denotes a weight for a jth piece of information transmitted through an ith Tx antenna. W is also referred to as a precoding matrix.
Given NR Rx antennas, signals received at the Rx antennas, y1, y2, . . . , yNR may be represented as the following vector.y=[y1, y2, . . . , yNR]T  [Equation 6]
When channels are modeled in the MIMO communication system, they may be distinguished according to the indexes of Tx and Rx antennas and the channel between a jth Tx antenna and an ith Rx antenna may be represented as hij. It is to be noted herein that the index of the Rx antenna precedes that of the Tx antenna in hij.
The channels may be represented as vectors and a matrix by grouping them. The vector representation of channels may be carried out in the following manner.
FIG. 8 illustrates channels from NT Tx antennas to an ith Rx antenna.
Referring to FIG. 8, the channels from the NT Tx antennas to the ith Rx antenna may be expressed as [Equation 7].hiT=└hi1, hi2, . . . , hiNT┘  [Equation 7]
Also, all channels from NT Tx antennas to NR Rx antennas may be expressed as the following matrix.
                    H        =                              [                                                  ⁢                                                                                h                    1                    T                                                                                                                    h                    2                    T                                                                                                ⋮                                                                                                  h                    i                    T                                                                                                ⋮                                                                                                  h                                          N                      R                                        T                                                                        ]                    =                      [                                                                                h                    11                                                                                        h                    12                                                                    …                                                                      h                                          1                      ⁢                                                                                          ⁢                                              N                        T                                                                                                                                                              h                    21                                                                                        h                    22                                                                    …                                                                      h                                          2                      ⁢                                                                                          ⁢                                              N                        T                                                                                                                                          ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                          i                      ⁢                                                                                          ⁢                      1                                                                                                            h                                          i                      ⁢                                                                                          ⁢                      2                                                                                        …                                                                      h                                          i                      ⁢                                                                                          ⁢                                              N                        T                                                                                                                                          ⋮                                                                                                                                          ⋱                                                                                                                                                                                          h                                                                  N                        R                                            ⁢                      1                                                                                                            h                                                                  N                        R                                            ⁢                      2                                                                                        …                                                                      h                                                                  N                        R                                            ⁢                                              N                        T                                                                                                                  ]                                              [                  Equation          ⁢                                          ⁢          8                ]            
Actual channels experience the above channel matrix H and then are added with Additive White Gaussian Noise (AWGN). The AWGN n1, n2, . . . , nNR NR Rx antennas is given as the following vector.n=[n1, n2, . . . , nNR]T  [Equation 9]
From the above modeled equations, the received signal is given as
                    y        =                              [                                                  ⁢                                                                                y                    1                                                                                                                    y                    2                                                                                                ⋮                                                                                                  y                    i                                                                                                ⋮                                                                                                  y                                          N                      R                                                                                            ]                    =                                                                      [                                                                                                              h                          11                                                                                                                      h                          12                                                                                            …                                                                                              h                                                      1                            ⁢                                                                                                                  ⁢                                                          N                              T                                                                                                                                                                                                                    h                          21                                                                                                                      h                          22                                                                                            …                                                                                              h                                                      2                            ⁢                                                                                                                  ⁢                                                          N                              T                                                                                                                                                                                          ⋮                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                            h                                                      i                            ⁢                                                                                                                  ⁢                            1                                                                                                                                                h                                                      i                            ⁢                                                                                                                  ⁢                            2                                                                                                                      …                                                                                              h                                                      i                            ⁢                                                                                                                  ⁢                                                          N                              T                                                                                                                                                                                          ⋮                                                                                                                                                                                          ⋱                                                                                                                                                                                                                                                            h                                                                                    N                              R                                                        ⁢                            1                                                                                                                                                h                                                                                    N                              R                                                        ⁢                            2                                                                                                                      …                                                                                              h                                                                                    N                              R                                                        ⁢                                                          N                              T                                                                                                                                                            ]                                [                                                                  ⁢                                                                                                    x                        1                                                                                                                                                x                        2                                                                                                                        ⋮                                                                                                                          x                        j                                                                                                                        ⋮                                                                                                                          x                                                  N                          T                                                                                                                    ]                            +                              [                                                                  ⁢                                                                                                    n                        1                                                                                                                                                n                        2                                                                                                                        ⋮                                                                                                                          n                        i                                                                                                                        ⋮                                                                                                                          n                                                  N                          R                                                                                                                    ]                                      =                          Hx              +              n                                                          [                  Equation          ⁢                                          ⁢          10                ]            
Meanwhile, the numbers of rows and columns of the channel matrix H representing a channel status are determined according to the number of Tx antennas and the number of Rx antennas, respectively. That is, the number of rows of the channel matrix H is equal to that of Tx antennas, NT and the number of columns of the channel matrix H is equal to that of Rx antennas, NR. Thus the channel matrix H may be expressed as an NR×NT matrix. In general, the rank of a matrix is determined to be the smaller between the number of independent rows and the number of independent columns in the matrix. Accordingly, the rank of the matrix is not larger than the number of rows or columns. For example, the rank of the channel matrix H, rank(H) is limited as follows.rank(H)≤min(NT,NR)  [Equation 11]
Reference Signals (RSs)
In a mobile communication system, a packet is transmitted on a radio channel. In view of the nature of the radio channel, the packet may be distorted during the transmission. To receive the signal successfully, the receiver should compensate for the distortion in the received signal using channel information. Generally, to enable the receiver to acquire the channel information, the transmitter transmits a signal known to both the transmitter and the receiver and the receiver acquires knowledge of channel information based on the distortion of the signal received on the radio channel. This signal is called a pilot signal or an RS.
According to the purposes that they serve, RSs are categorized into four types as listed in Table 1, Channel Quality Indicator-Common Reference Signal (CQI-CRS), Demodulation-Common Reference Signal (DM-CRS), Non-precoded Demodulation-Dedicated Reference Signal (NDM-DRS), and Precoded Demodulation-Dedicated Reference Signal (PDM-DRS).
TABLE 1RS TypesNotesCQI-CRSA common reference signal used for channel measurement.The UE determines a CQI, a Precoding Matrix Indicator(PMI), and a Rank Indicator (RI) based on CQI-CRSs.Thus, preferably, CQI-CRSs are uniformly distributedacross a total frequency band.DM-CRSAlthough a DM-CRS is a common reference signal used fordemodulation, it can be also used for channel measurement.Because a plurality of UEs use DM-CRSs for channelmeasurement, a precoding scheme for a specific UEcannot be applied to the DM-CRSs. Thus, when atransmitter precodes a Physical Downlink SharedCHannel (PDSCH), it needs to signal a used codebookon a Physical Downlink Control CHannel (PDCCH) toa receiver.NDM-DRSA non-precoded dedicated reference signal used fordemodulation.PDM-DRSA precoded dedicated reference signal used for modulation.The same precoding scheme is applied to the PDM-DRSand the PDSCH. Thus there is no need for signaling aused codebook on the PDCCH.
In case of data transmission and reception through multiple antennas, knowledge of channel states between Tx antennas and Rx antennas is required for successful signal reception. Accordingly, an RS should exist for each Tx antenna.
FIG. 9 illustrates a downlink RS allocation structure in case of a normal Cyclic Prefix (CP) in a 3GPP LTE system, and FIG. 10 illustrates a downlink RS allocation structure in case of an extended CP in the 3GPP LTE system. The downlink RS allocation structures illustrated in FIGS. 9 and 10 are for the current 3GPP LTE system.
Referring to FIGS. 9 and 10, the horizontal axis represents time and the vertical axis represents frequency in an RB. One subframe includes two slots. Each slot has seven OFDM symbols when the normal CP is used as illustrated in FIG. 9, whereas each slot includes six OFDM symbols when the extended CP is used as illustrated in FIG. 10. The extended CP is used generally under a long-delay environment. The RS allocation structures illustrated in FIGS. 9 and 10 are designed for four Tx antennas in a BS. Reference characters 0, 1, 2 and 3 denote CRSs for first to fourth antenna ports, antenna port 0 to antenna port 3, respectively, and reference character D denote DRSs.
As noted from FIGS. 9 and 10, in order to distinguish different antennas of a cell from one another, if an antenna port transmits an RS at an RE, the other antenna ports transmit no signals at the RE. Because channel estimation is performed using RSs, this scheme minimizes interference between antenna ports. When cell-specific RSs are used, the subcarrier spacing between RSs in a symbol is 6 and thus a cell-specific frequency shift value may range from 0 to 5. To avoid the same RS positions between cells, each cell determines RS positions using its cell-specific frequency shift value, with the aim to improve channel estimation performance by randomization of interference caused by RSs transmitted from other cells.
In the RS allocation structures illustrated in FIGS. 9 and 10, RSs are mapped to RBs according to the following rules described as [Equation 12] to [Equation 15]. Specifically, [Equation 12] and [Equation 13] describe the rule of mapping CRSs to RBs, and [Equation 14] and [Equation 15] describe the rule of mapping DRSs to RBs.
                              k          =                                    6              ⁢                                                          ⁢              m                        +                                          (                                  v                  +                                      v                    shift                                                  )                            ⁢              mod              ⁢                                                          ⁢              6                                      ⁢                                  ⁢                  l          =                      {                                                                                                                                                        0                          ,                                                                                    N                              symb                                                              D                                ⁢                                                                                                                                  ⁢                                L                                                                                      -                            3                                                                                                                                                                            if                            ⁢                                                                                                                  ⁢                            p                                                    ∈                                                      {                                                          0                              ,                              1                                                        }                                                                                                                                                              1                                                                                                                          if                            ⁢                                                                                                                  ⁢                            p                                                    ∈                                                      {                                                          2                              ,                              3                                                        }                                                                                                                                ⁢                                                                          ⁢                  m                                =                0                            ,              1              ,              …              ,                                                                    2                    ·                                          N                                              R                        ⁢                                                                                                  ⁢                        B                                                                    D                        ⁢                                                                                                  ⁢                        L                                                                              -                                      1                    ⁢                                                                                  ⁢                                          m                      ′                                                                      =                                                      m                    +                                          N                                              R                        ⁢                                                                                                  ⁢                        B                                                                    max                        ,                                                  D                          ⁢                                                                                                          ⁢                          L                                                                                      -                                                                  N                                                  R                          ⁢                                                                                                          ⁢                          B                                                                          D                          ⁢                                                                                                          ⁢                          L                                                                    ⁢                                                                                          ⁢                      v                                                        =                                      {                                                                                            0                                                                                                                                    if                              ⁢                                                                                                                          ⁢                              p                                                        =                                                                                          0                                ⁢                                                                                                                                  ⁢                                and                                ⁢                                                                                                                                  ⁢                                l                                                            =                              0                                                                                                                                                                            3                                                                                                                                    if                              ⁢                                                                                                                          ⁢                              p                                                        =                                                                                          0                                ⁢                                                                                                                                  ⁢                                and                                ⁢                                                                                                                                  ⁢                                l                                                            ≠                              0                                                                                                                                                                            3                                                                                                                                    if                              ⁢                                                                                                                          ⁢                              p                                                        =                                                                                          1                                ⁢                                                                                                                                  ⁢                                and                                ⁢                                                                                                                                  ⁢                                l                                                            =                              0                                                                                                                                                                            0                                                                                                                                    if                              ⁢                                                                                                                          ⁢                              p                                                        =                                                                                          1                                ⁢                                                                                                                                  ⁢                                and                                ⁢                                                                                                                                  ⁢                                l                                                            ≠                              0                                                                                                                                                                                                        3                            ⁢                                                          (                                                                                                n                                  s                                                                ⁢                                                                                                                                  ⁢                                mod                                ⁢                                                                                                                                                                          ⁢                                                                                                                                                                        ⁢                                2                                                            )                                                                                                                                                                                                                          if                                ⁢                                                                                                                                  ⁢                                p                                                            =                              2                                                        ⁢                                                                                                                                                                                                                                                                  3                            +                                                          3                              ⁢                                                              (                                                                                                      n                                    s                                                                    ⁢                                                                                                                                          ⁢                                  mod                                  ⁢                                                                                                                                                                                    ⁢                                                                                                                                                                                  ⁢                                  2                                                                )                                                                                                                                                                                                                                                        if                                ⁢                                                                                                                                  ⁢                                p                                                            =                              3                                                        ⁢                                                                                                                                                                                                                                                                                            [                  Equation          ⁢                                          ⁢          12                ]                                          v          shift                =                              N                          I              ⁢                                                          ⁢              D                        cell                    ⁢          mod          ⁢                                          ⁢          6                                    [                  Equation          ⁢                                          ⁢          13                ]                                                                                    k                =                                                                            (                                              k                        ′                                            )                                        ⁢                    mod                    ⁢                                                                                  ⁢                                          N                      sc                                              R                        ⁢                                                                                                  ⁢                        B                                                                              +                                                            N                      sc                                              R                        ⁢                                                                                                  ⁢                        B                                                              ·                                          n                                              P                        ⁢                                                                                                  ⁢                        R                        ⁢                                                                                                  ⁢                        B                                                                                                                        normal                ⁢                                                                  ⁢                C                ⁢                                                                  ⁢                P                                                                                        k                =                                                                            (                                              k                        ′                                            )                                        ⁢                    mod                    ⁢                                                                                  ⁢                                          N                      sc                                              R                        ⁢                                                                                                  ⁢                        B                                                                              +                                                            N                      sc                                              R                        ⁢                                                                                                  ⁢                        B                                                              ·                                          n                                              P                        ⁢                                                                                                  ⁢                        R                        ⁢                                                                                                  ⁢                        B                                                                                                                        extended                ⁢                                                                  ⁢                C                ⁢                                                                  ⁢                P                                                                                                        k                ′                            =                              {                                                                                                                              4                          ⁢                                                                                                          ⁢                                                      m                            ′                                                                          +                                                  v                          shift                                                                                                                                                              if                          ⁢                                                                                                          ⁢                          l                                                ∈                                                  {                                                      2                            ,                            3                                                    }                                                                                                                                                                                                  4                          ⁢                                                                                                          ⁢                                                      m                            ′                                                                          +                                                                              (                                                          2                              +                                                              v                                shift                                                                                      )                                                    ⁢                          mod                          ⁢                                                                                                          ⁢                          4                                                                                                                                                              if                          ⁢                                                                                                          ⁢                          l                                                ∈                                                  {                                                      5                            ,                            6                                                    }                                                                                                                                                                                            k                ′                            =                              {                                                                                                                              3                          ⁢                                                                                                          ⁢                                                      m                            ′                                                                          +                                                  v                          shift                                                                                                                                                              if                          ⁢                                                                                                          ⁢                          l                                                =                        4                                                                                                                                                                          3                          ⁢                                                                                                          ⁢                                                      m                            ′                                                                          +                                                                              (                                                          2                              +                                                              v                                shift                                                                                      )                                                    ⁢                          mod                          ⁢                                                                                                          ⁢                          3                                                                                                                                                              if                          ⁢                                                                                                          ⁢                          l                                                =                        1                                                                                                                                                                    l              =                              {                                                                            3                                                                                                                l                          ′                                                =                        0                                                                                                                        6                                                                                                                l                          ′                                                =                        1                                                                                                                        2                                                                                                                l                          ′                                                =                        2                                                                                                                        5                                                                                                                l                          ′                                                =                        3                                                                                                                                                    l              =                              {                                                                            4                                                                                                                l                          ′                                                ∈                                                  {                                                      0                            ,                            2                                                    }                                                                                                                                                1                                                                                                                l                          ′                                                =                        1                                                                                                                                                                                    l                ′                            =                              {                                                                                                    0                        ,                        1                                                                                                                                      if                          ⁢                                                                                                          ⁢                                                      n                            s                                                    ⁢                                                                                                          ⁢                          mod                          ⁢                                                                                                          ⁢                          2                                                =                        0                                                                                                                                                2                        ,                        3                                                                                                                                      if                          ⁢                                                                                                          ⁢                                                      n                            s                                                    ⁢                                                                                                          ⁢                          mod                          ⁢                                                                                                          ⁢                          2                                                =                        1                                                                                                                                                                    l                ′                            =                              {                                                                            0                                                                                                                if                          ⁢                                                                                                          ⁢                                                      n                            s                                                    ⁢                                                                                                          ⁢                          mod                          ⁢                                                                                                          ⁢                          2                                                =                        0                                                                                                                                                1                        ,                        2                                                                                                                                      if                          ⁢                                                                                                          ⁢                                                      n                            s                                                    ⁢                                                                                                          ⁢                          mod                          ⁢                                                                                                          ⁢                          2                                                =                        1                                                                                                                                                                                                      m                  ′                                =                0                            ,              1              ,              …              ⁢                                                          ,                                                3                  ⁢                                                                          ⁢                                      N                                          R                      ⁢                                                                                          ⁢                      B                                                              P                      ⁢                                                                                          ⁢                      D                      ⁢                                                                                          ⁢                      S                      ⁢                                                                                          ⁢                      C                      ⁢                                                                                          ⁢                      H                                                                      -                1                                                                                                          m                  ′                                =                0                            ,              1              ,              …              ⁢                                                          ,                                                4                  ⁢                                                                          ⁢                                      N                                          R                      ⁢                                                                                          ⁢                      B                                                              P                      ⁢                                                                                          ⁢                      D                      ⁢                                                                                          ⁢                      S                      ⁢                                                                                          ⁢                      C                      ⁢                                                                                          ⁢                      H                                                                      -                1                                                                        [                  Equation          ⁢                                          ⁢          14                ]                                          v          shift                =                              N                          I              ⁢                                                          ⁢              D                        cell                    ⁢          mod          ⁢                                          ⁢          3                                    [                  Equation          ⁢                                          ⁢          15                ]            
where vshift denotes a frequency shift value, k denotes a subcarrier index, P denotes an antenna port index, NRBDL denotes the number of allocated downlink RBs, ns denotes a slot index, and NIDcell denotes a cell ID.
According to [Equation 13] and [Equation 15], RSs of a cell may be shifted along the frequency axis by a frequency shift value vshift specific to the cell.
CoMP System
It is known that Inter-Cell Interference (ICI) generally degrades the performance of a UE at a cell edge. To minimize the performance degradation, a simple ICI mitigation technique is used, such as UE-specific Fractional Frequency Reuse (FFR) in LTE. However, an LTE-Advanced (LTE-A) system considers a technique for improving the performance of UEs at a cell edge by controlling ICI with coordination among a plurality of BSs. A system that supports one UE in coordination of a plurality of cells is called a Coordinated Multi-Point (CoMP) system in the standardization phase of LTE-A. The CoMP is characterized in that two or more BSs or cells coordinate with one another to improve the communication performance between a BS (cell or sector) and a UE in a shadowing area.
The CoMP system may increase the throughput of a UE at a cell edge by advanced MIMO transmission in a multi-cell environment. The CoMP system offers the benefits of ICI mitigation in the multi-cell environment and joint data support of multi-cell BSs for a UE. In addition, system performance may be improved since the BSs support one or more UEs (e.g. MS 1 to MS K) in the same radio frequency resources. The BSs may also operate in Space Division Multiple Access (SDMA) based on channel information between the UE and the BSs.
Depending on sharing of data and channel information among cells participating in a CoMP operation (hereinbelow, referred to as CoMP cells), different CoMP schemes are available.
Scheme 1. Both channel information and data are shared among the CoMP cells.
Scheme 2. Only channel information is shared among the CoMP cells.
Scheme 3. Only data is shared among the CoMP cells.
Scheme 4. Neither data nor channel information is shared among the CoMP cells.
Scheme 1 may improve the performance of a UE at a cell edge significantly, as data and channel information are shared among a serving cell and neighbor cells. However, UEs of the CoMP cells should feed back too a large amount of information, which makes it difficult to implement Scheme 1 in a real system. Moreover, the sharing of channel information among the neighbor cells may lead to a long delay.
In Scheme 2, the serving cell receives feedback channel information from UEs at cell edges in neighbor cells and mitigates ICI in a closed loop. While this scheme is considered as offering a gain with a minimal backhaul overhead, the feedback overhead of the UEs may impose a constraint on the system.
Scheme 3 may achieve a gain through open-loop transmission, minimizing system complexity.
Since any particular information for ICI mitigation is not shared among the serving cell and the neighbor cells, Scheme 4 enables simple ICI mitigation and is expected to give a marginal gain to the current LTE system.
Physical Downlink Control Channel (PDCCH)
The PDCCH carries control information about downlink data or uplink data. A UE determines whether there is downlink data directed to the UE or whether the UE is allowed to transmit uplink data, by monitoring the PDCCH. In general, the PDCCH is transmitted in every subframe and the UE determines whether the PDCCH is for the UE by running a random function using its unique ID.
Information transmitted on the PDCCH is divided into control information about downlink data and control information about uplink data. The control information about downlink data includes resource allocation information, modulation and coding information, HARQ process information, a New data Indicator (NDI), Redundancy Version (RV) information, power control information, and additionally, precoding information when MIMO is supported. Different control information about downlink data may be defined according to an operation mode.
The control information about uplink data includes resource allocation information, demodulation-RS resource information, CQI transmission request information, and additionally, precoding information when MIMO is supported.
The LTE-A standardization working body is considering various techniques for further improving the performance of a UE at a cell edge by CoMP-based ICI mitigation. Among them, a joint processing/transmission scheme transmits one or more data streams from a plurality of cells to one UE, sharing one Physical Resource Block (PRB) among the cells. The CoMP cells have their cell-specific frequency shift values for CRSs and DRSs and different numbers of antennas. This means that the CoMP cells have different RS allocation patterns. As a consequence, REs each carrying both data and an RS exist in a PRB allocated to the UE. If CRSs and DRSs reside together in a symbol, it may occur that CRSs overlap with DRSs according to the frequency shift values vshift of the cells. Consequently, system performance is degraded.